Turkey and mash pie

Name: Turkey and mash pie
Ingredients: 50 g butter; 1 large onion, finely chopped; 2 celery sticks, finely sliced; 3 garlic cloves, crushed; 25 g plain flour; 400 mL chicken stock; 200 mL white wine; 200 g cooked skinless turkey, shredded; 150 g mixed roasted vegetables (we used carrots and parsnips), roughly chopped; 125 g frozen peas; small handful fresh parsley, roughly chopped; 750 g floury potatoes, peeled and cut into even chunks; 50 mL milk; 2 tbsp. wholegrain mustard; 2 medium egg yolks